1. Field of the Invention
The present invention relates to a ceramic heater used in a ceramic glow plug attached to a diesel engine or the like.
2. Description of the Related Art
A conventional ceramic heater for a ceramic glow plug attached to a diesel engine is composed of a bar-shaped insulating ceramic heater body, a metallic sleeve fitted onto the ceramic heater body, a resistance heating element formed of a metal or a nonmetallic material and embedded in the ceramic heater body, and electrode leads. Such conventional ceramic heaters can be divided into two types, which differ according to the structure employed for establishing connection between the electrode lead of a ceramic heater and an intermediate shaft having one end fixedly held within a metallic sleeve of a ceramic glow plug. In a ceramic heater of one type, a temperature control resistor is interposed between the intermediate shaft of the glow plug and a lead coil connected to the electrode lead of the ceramic heater. In a ceramic heater of the other type, the intermediate shaft of the glow plug is connected directly to the lead coil.
In the ceramic heater in which a temperature control resistor is interposed between the intermediate shaft of the glow plug and a lead coil connected to the electrode lead of the ceramic heater, the temperature control resistor allows the embedded resistance heating element to quickly increase its temperature, to thereby generate a sufficient amount of heat for starting an engine. However, since the temperature control resistor must be incorporated within the metallic shell, the manufacturing cost increases, resulting in an expensive ceramic glow plug.
By contrast, in the ceramic heater in which the intermediate shaft of the glow plug is connected directly to the lead coil, the above-mentioned quick temperature increase achieved by the embedded resistant heating element is not expected. Since no temperature control resistor is used, the structure for establishing connection between the intermediate shaft and the ceramic heater is simple. However, in order to impart sufficient engine starting performance to a ceramic glow plug utilizing such a ceramic heater, the following point must be considered in design of the ceramic heater. That is, measures for generating a sufficient amount of heat through a quick temperature increase include raising the saturation temperature of the resistance heating element greatly or employing a controller for controlling application voltage. However, when the saturation temperature of the resistance heating element is increased excessively, the durability of the ceramic heater itself decreases. When a controller for controlling application voltage is employed, the complicated structure of the controller considerably increases the overall cost of the product.